The present invention generally relates to prosthetic implants, and more particularly to such implants which use projections or spacers to maintain a uniform layer of bone cement between the prosthesis and the bone to which the prosthesis is to be secured.
Prosthetic devices for replacement of damaged or deteriorating portions of bone are well known in the art. Prosthetic devices for replacement of damaged or deteriorating portions of bone are well known in the art. Prosthetic devices are often secured to the bone with a layer of bone cement. In order to provide a more secure fixation of the prosthesis a uniform, even layer of cement is desired over the entire area of fixation of the prosthesis to the bone.
The acetabulum is one area where this even layer of cement is of particular concern. When both parts of the ball and socket hip joint need replacement, the femoral head is generally replaced with a prosthetic ball-shaped member and the acetabulum is generally lined with a socket or cup member adapted for receiving the ball-shaped member. Various types of spacers have been used to prevent the cup from "bottoming out" of the acetabular bone when cement is used to secure the cup. When spacer devices are not used, the surgeon may apply a layer of cement to the acetabular bone and then place the cup in position against the bone cement. In pressing the cup into place in the cement, the surgeon may cause the cement to unevenly distribute. For example, the cement tends to squeeze out around the edges of the cup, often creating a very thin layer of cement at the portion directly behind the center of the back of the cup, while the cement is much thicker around the edges. This creates a weak securing of the prosthesis.
Prior art devices utilizing spacers include:
U.S. Pat. No. 4,285,071 to Nelson, et al, discloses the use of a plurality of separate or self-contained spacers for use with prosthetic devices, such as acetabular cups. These separate spacers provide a means for controlling uniformity and thickness of the cement applied between the prosthetic device and its supporting bone member. Each separate spacer include a standoff body with a pointed wire outwardly extending therefrom. These spacers are individually positioned by inserting the wire into the bone member. The bone cement layer is then applied, and the prosthetic device member is then placed against the outermost surface of each of standoff bodies. The standoff body of the individual spacers may be made out of acrylic bone cement. The surface of the polymerized acrylic bone cement of the standoff body portion repolymerizes with the new acrylic bone cement layer when it is introduced. The nature of the bond formed between the new acrylic cement layer with the standoff body helps to eliminate stress concentrations at the interface. The disadvantage of the separate spacers is that they have to be individually positioned by the surgeon. This is not only time consuming, but allows for error in positioning of the spacers in relation to each other and the prosthetic device which is to be positioned on the spacers. Also, after the repolymerization, the sharp "thumb tack" like pointed wires remain.
U.K. Patent Application No. GB 2 080 118 A to Hardinge discloses an acetabular cup made out of a plastic material. The external surface of the cup includes a plurality of studs made of the same material as the remainder of the cup. The studs may be either integral with the cup or inserted into the cup. The studs act as spacers to facilitate the formation of a layer of bone cement of substantially constant thickness between the cup and the acetabulum. Spacers such as those of Hardinge which are made of plastic do not incorporate themselves into the cement, but instead they may tend to compartmentalize the cement. Also, plastic spacers may tend to create a potential for stress risers in the surrounding bone cement.
Another prior art acetabular cup is the ES-32 cup sold by Biomet. This cup includes a plurality of protruding spacers to facilitate the formation of a uniform thickness of bone cement to eliminate thick and thin sections of bone cement. The spacers are integrally molded with the polyethylene cup. The cup is reinforced with a metal back cap to guard against deformation and potential resultant cement fixation breakdown caused by excessive cold flow. Openings are provided in the metal cap for the spacers to protrude through. As with the spacers of Hardinge, the plastic spacers of the ES-32 do not incorporate themselves into the bone cement, but instead they may tend to compartmentalize the cement, and might even create stress risers in the surrounding cement.